Methods of nanostructure formation and shape selection

ABSTRACT

Methods for forming nanostructures of various shapes are disclosed. Nanocubes, nanowires, nanopyramids and multiply twinned particles of silver may by formed by combining a solution of silver nitrate in ethylene glycol with a solution of poly(vinyl pyrrolidone) in ethylene glycol. Hollow nanostructures may be formed by reacting a solution of solid nanostructures comprising one of a first metal and a first metal alloy with a metal salt that can be reduced by the first metal or first metal alloy. Nanostructures comprising a core with at least one nanoshell may be formed by plating a nanostructure and reacting the plating with a metal salt.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/919,515 filed Oct. 21, 2015, which is a continuation of U.S.application Ser. No. 12/509,873 filed Jul. 27, 2009, which is adivisional of U.S. patent application Ser. No. 10/732,910, filed on Dec.9, 2003, which claims the benefit of U.S. Provisional Application No.60/432,098, filed on Dec. 9, 2002. The entire contents of each of theforegoing applications are hereby incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was developed under support from the Office of NavalResearch (grant N-0014-01-1-0976), “Self-Assembly Approaches to 3DPhotonic Crystals” and a Career Award from NSF (grant DMR-9983893),“Nanostructured Surfaces and Materials”; the Government has certainrights in the invention.

BACKGROUND OF THE INVENTION

Metal nanoparticles play important roles in many different areas. Forexample, they can serve as a model system to experimentally probe theeffects of quantum-confinement on electronic, magnetic, and otherrelated properties. They have also been widely exploited for use inphotography, catalysis, biological labeling, photonics, optoelectronics,information storage, surface-enhanced Raman scattering (SERS), andformulation of magnetic ferrofluids. The intrinsic properties of a metalnanoparticle are mainly determined by its size, shape, composition,crystallinity, and structure (solid versus hollow). In principle, anyone of these parameters can be controlled to fine-tune the properties ofthis nanoparticle. For example, the plasmon resonance features of goldor silver nanorods have been shown to have a strong dependence on theaspect-ratios of these nanostructures. The sensitivity ofsurface-enhanced Raman scattering (SERS) has also been demonstrated todepend on the exact morphology of a silver nanoparticle.

Many metals can now be processed into monodisperse nanoparticles withcontrollable composition and structure, and sometimes can be produced inlarge quantities through solution-phase methods. Despite this, thechallenge of synthetically controlling the shape of metal nanoparticleshas been met with limited success. On the nanometer scale, metals (mostof them are face-centered cubic (“fcc”)) tend to nucleate and grow intotwinned and multiply twinned particles with their surfaces bounded bythe lowest-energy {111} facets. Other morphologies with less stablefacets have only been kinetically achieved by adding chemical cappingreagents to the synthetic systems. For examples, triangular nanoplatesof gold have been synthesized by reducing chloroauric acid with citricacid (rather than sodium citrate) and by adding sodium hydroxidesolution toward the end of this reaction. Silver nanoprisms in largequantities have also been prepared through a photo-induced approach inwhich small silver nanospheres transform to nanoprisms with the help ofcitrate and a co-ligand such as bis(p-sulfonatophenyl) phenylphosphinedehydrate dipotassium.

When a metal nanostructure is processed into a hollow entity, itsperformance can be further improved due to its relatively lower densityand higher surface area than its solid counterpart. For instance, hollownanoshells made of palladium have been shown to be an effective, wellrecoverable catalyst for Suzuki coupling reactions, while themonodisperse solid palladium nanoparticles greatly lose their catalyticability after a single use.

Hollow nanostructures made of metals can be fabricated by depositing athin layer of metal (or its precursor) on an existing solidnanostructure (e.g., silica beads and polymeric latexes) followed withthe calcinations or chemical etching to remove the templates. However, aprocedure for manufacturing hollow nanostructures with smooth, nonporoussurfaces, homogenous, highly crystalline walls and structural integrityis needed.

Additional aspects and advantages of this invention will be apparentfrom the following detailed description of preferred embodiments, whichproceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a silver nanocube manufactured by themethods described herein with facets {100}, {111} and {110}.

FIGS. 2A and 2B show low- and high-magnification SEM images of slightlytruncated silver nanocubes synthesized with the present approach.

FIG. 2C shows a TEM image of the same batch of silver nanocubes shown inFIGS. 2A and 2B. The inset shows the electron diffraction patternobtained by directing the electron beam perpendicular to one of thesquare faces of a cube.

FIG. 2D shows an XRD pattern of the same batch of sample shown in FIGS.2A-2C.

FIGS. 3A and 3B show TEM images of silver nanocubes synthesized underthe same conditions as in FIGS. 2A-2D except that the growth time was 17minutes for FIG. 3A and 14 minutes for FIG. 3B.

FIG. 4 shows a TEM image of silver nanocubes synthesized under the sameconditions as in FIGS. 2A-2D except that the concentration of silvernitrate was 0.125 mol/dm³ and the growth time was 30 min.

FIG. 5 shows an SEM image of nanopyramids formed under the sameconditions as the nanocubes of FIGS. 2A-2C except that the reactiontemperature was decreased to 100° C. and the growth time elongated to 5hours.

FIG. 6A shows an SEM image of silver nanocubes, such as those shown inFIGS. 2A-2D, following reaction with an insufficient amount of aqueoussolution of chloroauric acid.

FIG. 6B shows electron diffraction patterns of two gold/silver alloyednanoboxes with their square and triangular facets oriented perpendicularto the electron beam, respectively.

FIGS. 6C and 6D show electron diffraction patterns of individualnanoboxes sitting on TEM grids against one of their square andtriangular faces, respectively.

FIG. 7A is a diagram of the procedure for manufacturing nanoboxes fromsilver nanocubes.

FIG. 7B is a diagram of the reaction that occurs in the replacementprocess.

FIG. 8 is a schematic illustration of the formation of movable solidcore (Au/Ag alloy) and shell (Au/Ag alloy).

FIG. 9A shows a schematic illustration of the formation of a multiplewalled hollow nanoshell.

FIG. 9B shows TEM images of nanostructures, each with a coreencapsulated by a single nanoshell.

FIG. 9C shows TEM images of nanostructures, each with a coreencapsulated by a double nanoshell.

FIG. 9D shows TEM images of nanostructures, each with a coreencapsulated by a triple nanoshell.

FIG. 10 is a schematic illustration of the formation of hollownanotubes.

FIG. 11A shows an SEM image sample of silver nanocubes.

FIGS. 11B-11G and 11J-11M show SEM images of samples following thereaction silver nanocubes with differing amounts of HAuCl₄.

FIGS. 11H and 11I show electron diffraction patterns of two nanoboxes.

FIGS. 12A-12F show TEM images of samples before (FIG. 12A) and followingthe reaction silver nanowires with differing amounts of HAuCl₄ solution(FIGS. 12B-12F). The insets are SEM images.

FIGS. 13A-13F show TEM images obtained from spherical silvernanoparticles before (FIG. 13A) and following reaction with differingamounts of HAuCl₄ solution (FIGS. 13B-13F). The insets are SEM images.

FIG. 14 shows a TEM image of Pt nanotubes formed by a method of thepresent invention.

FIG. 15A shows another TEM image of Pd nanotubes.

FIG. 15B shows a typical SEM image of sonicated nanotubes, indicatingthat their surfaces were still continuous and smooth.

FIG. 16A shows SEM images and FIG. 16B shows SAED patterns of silvernanowires.

FIG. 16C shows SEM images and FIG. 16D shows SAED patterns ofsingle-shelled nanotubes of Au/Ag alloy.

FIG. 16E shows SEM images and FIG. 16F shows SAED patterns ofdouble-shelled nanotubes of Au/Ag alloy.

FIG. 17 shows an SEM image of triple-shelled nanotubes of Au/Ag alloy.

FIG. 18 shows an SEM image of double shelled nanotubes with theconstituent material of inner shell being Au/Ag alloy and the outershell being Pd/Ag alloy.

DETAILED DESCRIPTION

I. The Formation of Silver Nanostructures

Silver nanostructures of various shapes can be formed by the reductionof silver nitrate with ethylene glycol in the presence of poly(vinylpyrrolidone) (“PVP”). The morphology and dimensions of the productdepend on reaction conditions, including temperature, the concentrationof silver nitrate, and the molar ratio between the repeating unit of PVPand silver nitrate. The methods described herein provide nanostructureswith high uniformity in sizes, controllable size and shape, singlecrystallinity, large quantities, and good dispersibility in variablesolvents. Uniform hollow nanostructures can be generated by reacting thesilver nanostructures with other metal precursors, such as chloroauricacid (HAuCl₄), paladium nitrate (Pd(NO₃)₂) or platinum acetate(Pt(CH₃COO)₂. The formation of hollow nanostructures is discussed inmore detail below. Nanostructures surrounded by a shell (a rattle-likestructure) may be formed by plating solid or hollow nanostructures andreacting the plating with another metal salt. The generation of thecore/shell structures is discussed in more detail below.

FIG. 1 is an illustration of a silver nanocube generated by the methodsdescribed herein with facets {100}, {111} and {110}. FIGS. 2A and 2Bshow SEM images of a typical sample of silver nanocubes obtained byadding silver nitrate in ethylene glycol at a concentration of 0.25mol/dm³ and PVP in ethylene glycol at a concentration of 0.375 mol/dm³to heated etheylene glycol and allowing the reaction to proceed at areaction temperature of 160° C. The injection time was 8 min, the unitof volume was one milliliter (mL) and the reaction time was 45 minutes.The formation of silver nanocubes is described in more detail inconnection with Example 1 below.

FIGS. 2A and 2B indicate the large quantity and good uniformity thatwere achieved. The silver nanocubes had a mean edge length of 175 nmwith a standard deviation of 13 nm. Their surfaces were smooth,nonporous and some of them self-assembled into ordered two-dimensionalarrays on the silicon substrate when the scanning electron microscopy(“SEM”) sample was prepared. All corners and edges of the nanocubes wereslightly truncated, as can be seen in FIG. 2B. The image shown in FIG.2B was taken at a tilting angle of 20°. FIG. 2C shows a transmissionelectron microscopy (“TEM”) image of an array of silver nanocubesself-assembled on the surface of a TEM grid. The inset shows theelectron diffraction pattern obtained by directing the electron beamperpendicular to one of the square faces of a cube. The square symmetryof this pattern indicates that each silver nanocube was a single crystalbounded mainly by {100} facets. On the basis of these SEM and TEMstudies, it is clear that the slightly truncated nanocube can bedescribed by the drawing shown in FIG. 1.

FIG. 2D shows the x-ray diffraction (XRD) pattern recorded from the samebatch of sample shown in FIGS. 2A-2C. The peaks were assigned todiffraction from planes (111), (200) and (220) of fcc silver nanocubes.The ratio between the intensities of 200 and 111 diffraction peaks washigher than the conventional value (0.67 versus 0.4), indicating thatnanocubes manufactured as described herein are abundant in {100} facets.Thus their (100) planes tend to be preferentially oriented (or textured)parallel to the surface of the supporting substrate. The ratio betweenthe intensities of (220) and (111) peaks was also slightly higher thanthe conventional value (0.33 versus 0.25) due to the relative abundanceof {110} facets on the surfaces of our silver nanocubes.

The dimensions of silver nanostructures could be controlled by varyinggrowth time. The silver nanocubes described above in connection withFIGS. 2A-2D were formed over a reaction time of 45 minutes had anaverage edge length of 175 nm.

FIGS. 3A and 3B show TEM images of silver nanocubes synthesized underthe same conditions as the nanocubes shown in FIGS. 2A-2D except thatthe growth time was shortened from 45 to 17 minutes (FIG. 3A) and to 14minutes (FIG. 3B). A growth time of 17 minutes obtained nanocubes withaverage edge lengths of 115 nm. A growth time of 14 minutes resulted innanocubes with average edges lengths of 95 nm.

FIG. 4 shows a TEM image of silver nanocubes synthesized under the sameconditions as in FIGS. 2A-2D except that the concentration of silvernitrate had been reduced from 0.25 mol/dm³ to 0.125 mol/dm³ and thegrowth time had been shortened to 30 minutes. The average edge lengthwas 80 nm. These results demonstrate that it is possible to tune thesize of silver nanocubes by controlling the growth conditions.

Varying reaction conditions can select for nanostructures with differentshapes. Once a desired shape is identified, the nanostructures may beformed under condition optimized to yield the desired shape at a higherpercentage than any other nanostructure shape. The nanostructures of thedesired shape may then be separated from nanostructures of the othershapes by centrifugation (gravity) or by filtration.

Crystalline silver nanocubes may be obtained when under the followingreaction conditions: (1) the initial concentration of silver nitrate inethylene glycol ranges from about 0.1 mol/dm³ to about 0.3 mol/dm³; (2)the molar ratio of PVP to silver nitrate ranges from about 1.5 to about3; (3) the PVP used has a molecular weight ranging from about 40,000 toabout 1,300,000; (4) the growth time ranges from about 10 minutes toabout 60 minutes; and (5) reaction temperatures ranges from about 155°C. to about 175° C. These reaction conditions may depend on one another.For example, when the molar ratio of PVP to silver nitrate is 3, theconcentration of silver nitrate used to obtain silver nanocubes is 0.125mol/dm³. In another example, when the molar ratio is 1.5, theconcentration of silver nitrate used to obtain silver nanocubes is 0.25mol/dm³. Specific methods for generating different silver nanostructuresare discussed in more detail in connection with Examples 1-4 below.

Half-cubes or pyramids may be obtained by similar conditions: (1) asilver nitrate concentration of about 0.25 mol/dm³; (2) a ratio of PVPto silver nitrate of about 1 to about 4; (3) a PVP molecular weightranging from about 40,000 to about 1,300,000; (4) a reaction temperatureof about 90° C. to about 110° C.; and (5) a growth time of about 4 toabout 10 hours. FIG. 5 shows an SEM image of a sample of pyramids formedat a reaction temperature of about 100° C. and a growth time of 5 hours.

Silver nanowires may be obtained under the following reactionconditions: (1) a concentration of silver nitrate of less than about 0.1mol/dm³; (2) ratio of PVP to silver nitrate ranging from about 1 toabout 10; (3) PVP molecular weight ranging from about 20,000 to about1,300,000; (4) a reaction temperature ranging from about 150° C. toabout 190° C.; and (5) a growth time ranging from about 20 to about 60minutes.

Multiply twinned particles were obtained under the condition describedabove for forming nanocubes except that the molar ratio between therepeating unit of PVP and silver nitrate was increased from 1.5 to 3. Ifthe ratio of PVP to silver nitrate approaches 3, then a silver nitrateconcentration of about 0.25 mol/dm³ can be used to form multiply twinnedparticles.

Spherical silver nanoparticles were also synthesized using the polyolprocess. In a typical synthesis, 0.025 g silver nitrate (99+%, Aldrich)and 0.10 g PVP (molecular weight of about 55,000, Aldrich) weredissolved in 10 mL anhydrous ethylene glycol (99.8%, Aldrich) at roomtemperature. The mixture was heated at 160° C. for 1.5 hours while itwas vigorously stirred. The average diameter of these particles was 75nm. The diameter of the particles could be controlled by changing theconcentrations of silver nitrate and PVP. The amount of silver nitratefor forming spherical silver nanoparticles used in the volumes disclosedabove ranges from about 0.01 grams to about 1.0 grams. The amount of PVPfor forming spherical silver nanoparticles used in the volumes disclosedabove ranges from about 0.05 to about 2 grams. The diameter of silvernanospheres made by this process can be tuned in the range of about 20nm to about 300 nm.

The shape of an fcc nanocrystal is mainly determined by the ratio (R)between the growth rates along<100> and <111> directions. Octahedra andtetrahedra bounded by the most stable planes {111} will be formed whenR=1.73 and perfect cubes bounded by the less stable planes {100} willresult if R is reduced to 0.58. For the slightly truncated nanocubeillustrated in FIG. 1, the ratio R should have a value close to about0.7. If PVP is not present, reducing silver nitrate with ethylene glycolforms multiply twinned particles bounded by the most stable {111}facets. When PVP is introduced, it is believed that the selectiveinteraction between PVP and various crystallographic planes of fccsilver could greatly reduce the growth rate along<100> and/or enhancethe growth rate along<111>, and thus reduce R from 1.73 to 0.7. BothFourier Transform Infrared Spectroscopy and X-Ray PhotoelectronSpectroscopy measurements indicate that there exists a stronginteraction between the surfaces of silver nanoparticles and PVP throughcoordination bonding with the O and N atoms of pyrrolidone ring, albeitthe exact bonding geometry and the nature of the selectivity betweendifferent crystallographic planes are still not clear.

The synthetic strategy presented here to prepare silver nanocubes shouldbe extendable to other metals since ethylene glycol can reduce a broadrange of metallic salts to generate metals, including noble metals(e.g., gold, platinum, palladium and copper), magnetic metal (e.g.,iron, cobalt and nickel) and some superconductive metals (e.g., lead).The major requirement seems to be the availability of an appropriatepolymer that will be capable of forming coordination compounds withthese metal ions and can selectively adsorb onto different surfaces ofthese metals.

II. Formation of Hollow Nanostructures

Hollow nanostructures of other metals, such as gold/silver,platinum/silver and palladium/silver alloys, may be formed by usingsilver nanostructures or nanostructures of other metals as sacrificialtemplates. This method yields hollow nanostructure with a singlemanufacturing step. The hollow nanostructures can be selected to havesubstantially nonporous walls or can be selected to have porous walls.If desired, the hollow nanostructures yielded by this method can alsohave smooth, nonporous surfaces, homogenous, highly crystalline wallsand structural integrity. There are two requirements in obtaining ahollow metal nanostructure of a particular metal or alloy: the propersolvent and a salt that can be reduced by the nanometer-sized templates.For example, silver nanocubes may be used as sacrificial templates togenerate gold/silver alloy nanoboxes with a well-defined shape andhollow structure, based on the following reaction:3Ag(s)+HAuCl₄(aq)→Au(s)+3AgCl(aq)+HCl(aq).

Based on this stoichiometric relationship, silver nanocubes, forexample, can be converted into soluble species and leave behind agold/silver alloy in the form of nanoboxes. The resulting structures maybe selected to be nanoboxes with solid walls or nanocages with porouswalls, depending on the amount of HAuCl₄ added to the silver nanocubes.

In one embodiment, the method of preparing hollow nanostructurescomprises: (1) obtaining a solution of solid nanostructures comprisingat least one metal; (2) selecting a salt of a second metal, wherein thefirst metal can reduce the salt; (3) blending a sufficient amount of thesalt with the solid nanostructure solution to enable the formation ofhollow nanostructures. These hollow nanostructures are formed by areplacement reaction between the metal salt and the metal in the solidnanostructure.

Two successive, distinctive processes are involved in this replacementreaction. The first process involves the combination of dissolution ofsilver templates and alloying between deposited gold layers and silver,together with the formation of seamless nanostructures that have hollowinteriors and uniform walls composed of gold/silver alloy. The secondprocess involves the dealloying, which is associated with themorphological reconstruction as well as the generation of pinholes inthe walls. For example, in the first step of the reaction between silvernanocubes and HAuCl₄ solution, the silver nanocubes were transformedinto pinhole-free nanoboxes. In the second step, the dealloying processselectively dissolved silver atoms of the Au/Ag wall and latticevacancies were generated in the wall. The Ostwald ripening process couldrearrange these lattice defects, resulting in the formation of truncatednanoboxes and porous nanoboxes (nanocages).

FIG. 6A shows an SEM image of a sample of 5 mL of solution containingsilver nanucubes at a concentration of about 4×10⁹ particles/mL afterreaction with 0.3 mL of aqueous 1×10⁻³ mol/dm³ HAuCl₄ solution, whichwas an insufficient amount of HAuCl₄. The black spots represent pinholesin their surfaces where no gold had been deposited through thereplacement reaction. It is believed that the existence of such pinholesallows for the transport of chemical species into and out of thegold/silver boxes until the boxes became seamless. The locations ofthese black spots implied that the replacement reaction occurred on thesurface of a template in the following order: {110}, {100}, and {111}facets. This sequence was consistent with the order of free energiesassociate with these crystallographic planes:Y_({110})>Y_({100})>Y_({111}). Additional SEM images showing theformation of nanoboxes by this process are discussed in connection withExample 8.

FIG. 6B shows an SEM image of sample of 5 mL of solution containingsilver nanucubes at a concentration of about 4×10⁹ particles/mLfollowing reaction with 1.5 mL of aqueous 1×10⁻³ mol/dm³ HAuCl₄solution. The gold/silver nanoboxes shown in FIG. 6B self-assembled intoa close packed two-dimensional array during sample preparation. The sizeof these gold boxes increased by about 20% compared with that of thesilver templates. This increase in size was in agreement with the shellthickness calculated from stoichiometric and geometric arguments. Thegold/silver nanoboxes were finished with smooth surfaces, and most ofthem (>95%) were free of irregularities such as pinholes. Statedotherwise, the gold/silver alloy nanoboxes were substantiallynon-porous. Each box was bounded by two sets of facets (eight triangularand six square ones), and any one of these facets could lie against asolid substrate. The inset of FIG. 6B shows the SEM image of anindividual box sitting on a silicon substrate against one of itstriangular facets, illustrating the high symmetry of this polyhedral,hollow nanoparticle. The crystallinity and structure of these nanoboxeswere examined using electron diffraction.

FIGS. 6C and 6D show electron diffraction of nanoboxes sitting on TEMgrids against one of their square and triangular faces, respectively.These diffraction spots suggest that each nanobox was a single crystal,with its square facets being indexed to {100} planes and triangular onesto {111} planes. These observations suggest that an epitaxialrelationship exists between the surfaces of the silver cubes and thoseof gold/silver alloyed boxes that greatly facilitated the transformationfrom the single crystalline templates to the single crystallineproducts. Minor reconstruction also occurred in the replacement process:for example, the {110} planes that were observed as ridges on thesurfaces of silver cubes disappeared and the areas of {111} and {100}facets were enlarged and reduced, respectively.

FIG. 7 is a schematic illustration of morphological and structuralchanges involved in the galvanic replacement reaction between a silvernanocube and an aqueous HAuCl₄ solution. The major steps can besummarized as the following: (A) initiation of the replacement at aspecific spot having relatively high surface energy; (B) continuation ofthe replacement reaction between Ag and HAuCl₄ along with the formationof a partially hollow structure; (C) formation of nanoboxes withuniform, smooth, nonporous, homogeneous walls composed of a Au/Ag alloy;(D) initiation of dealloying and morphological reconstruction of theAu/Ag nanobox; (E, F) continuation of dealloying, together with theformation of small pores in the walls; (G) fragmentation of the porousAu nanobox. The cross section represents the plan along the dashedlines.

Silver nanowires can react with HAuCl₄ and display a similarmorphological evolution process to the process describe above fornanoboxes. Reaction temperature plays a critical role in the replacementreaction because the dissolvability of AgCl and the diffusion of metalswere strongly dependent on temperature. This template-engagedreplacement reaction between silver nanostructures and other metalsenables the preparation of metal nanostructures with precisely designedgeometric constructions. Controlling the morphology of metalnanostructures provides an effective mean to tune their properties. Forexample, nanostructures with different morphologies formed by reactingthe same amount of silver nanocubes and different volumes of HAuCl₄solution could continuously tune their surface plasmon resonance peakover a broad range from 500 to 1200 nm.

Silver templates may be used to generate hollow structures by usingother metal ions that can be reduced by silver. For example,palladium/silver and platinum/silver hollow structures can be generatedby reacting their salt with silver templates. The reaction for reactingsilver with a palladium salt is:Pd(NO ₃)_(2(aq))+2Ag _((s)) →Pd _((s))+2AgNO _(3(ag))

The reaction for reacting silver to with a platinum salt is:Pt(CH ₃ COO)_(2(aq))+2Ag _((s)) →Pt _((s))+2Ag(CH ₃ COO)_((aq))

Alternatively, Ni/Co alloy nanoparticles may be used a sacrificialtemplates for forming hollow nanostructures of other metals or metalalloys. The replacement reaction is based on two equations, one for theconversion of nickel and one for the conversion of cobalt. For example,the two replacement reactions for generating silver or silver/alloyhollow nanoparticles using Ni/Co alloy nanoparticles are:Ni _((s))+2AgNO _(3(aq)) →Ni(NO ₃)_(2(aq))+2Ag _((s))andCo _((s))+2AgNO _(3(aq)) →CO(NO ₃)_(2(aq))+2Ag _((s)).

The two replacement reactions for generating gold or gold alloy hollownanostructures from Ni/Co alloy nanoparticles are:3Ni _((s))+2HAuCl_(4(aq))→3NiCl_(2(aq))+2HCl _((aq))+2Au _((s))and3Co _((s))+2HAuCl4_((aq))→3CoCl_(2(aq))+2HCl _((aq))+2Au _((s)).

Methods for generating hollow metal nanoparticles using a sacrificialtemplate are discussed in more detail below in connection with Examples5-13.

III. Complex Nanoshell and Nanotube Formation

Core/shell nanostructures (nanostructures having with cores encapsulatedby a nanoshell) can be made by preparing solid nanoparticles, coatingthe nanoparticles with a layer of a different metal or metal alloy andallowing the coating to be replaced with another metal or metal alloy.Alternatively, multiple walled hollow nanostructures may be formed byusing hollow nanostructures, manufactured as described above, as aprecursor. The nanostructure core and nanoshell are separated by a spacealong at least a portion of the circumference of the nanostructure core.In one embodiment, the nanostructure core is unattached to theencapsulating nanoshell and is can move freely within the nanoshell.

FIG. 8 is a schematic illustration of the formation of movable solidcore (gold/silver alloy) and shell (gold/silver alloy). Step (A) showsthe plating of a silver layer on the surface of an gold/silver alloyedsolid nanoparticle. Step (B) shows the plated silver layer reacting withHAuCl4, resulting in the formation of a rattle-like core/shellstructure.

Shell thickness and morphology can be controlled by controlling thevolume of HAuCl₄ solution added to the dispersion of nanostructures. SEMor TEM images may be taken following the addition of each drop todetermine whether the desired shell thickness and morphology has beenreached. The spacing between core and shell may also be tuned bychanging the concentration of AgNO3 in the silver plating step. Theextinction peaks exhibited by these rattle-like core/shell structuresmay also be tuned by controlling the reaction conditions.

These rattle-like core/shell nanostructures may be formed with more thanone shell surrounding the nanoparticles by repeating the plating andreplacement process with a metal salt differing from salt used to createthe previous shell. For example, a first nanoshell may be created byplating a Ag/Au allowed solid nanoparticles with silver, then reactingthe silver plating with HAuCl4. Additional nanoshell may be added byrepeating the process. Alternatively, additional nanoshells may begenerated by coating the core/shell particles with silver and thenadding a different metal salt, such as Pd(NO₃)₂ or Pt(CH₃COO)₂. FIG. 9Ashows a schematic illustration of the formation of a multiple walledhollow nanoshell. Step (I) represents the formation of a hollownanostructure as described above. Step (II) shows the plating of thehollow nanostructure and Step (III) represents the replacement reactionof the plating to form a shell. FIG. 9B shows a TEM image of ananostructures, each with a single nanoshell; FIG. 9C shows a TEM imageof nanostructures with double nanoshells, and FIG. 9D showsnanostructures with triple nanoshells formed by this process.

FIG. 10 is a schematic illustration of the formation of hollownanotubes. Step A shows a silver nanowire reacting with HAuCl₄ to form ahollow nanotube whose morphology is complementary to that of the silvernanowire. The resultant nanotube could be coated with a conformal, thinsheath of Ag through an electroless plating process, shown as Step B.After repeating the galvanic replacement reaction, a new tubular wallwith slightly larger lateral dimensions would be formed, and adouble-walled nanotube was obtained, Step C. Coaxial nanotubes with morethan two walls could be readily synthesized by repeating Steps B and C.The thickness of each new wall can be controlled by varying theconcentration of AgNO₃ solution used for plating silver layers. Thesenanotubes may serve as substrates for surface-enhanced Ramanspectroscopic (“SERS”) detection of molecular species withultra-sensitivity in the spectral region form red to near infrared(“NIR”), which happens to be a transparent window for biologicalsamples.

EXAMPLES Examples 1-4 Formation of Silver Nanostructures Example 1

Example 1: A 3-neck glass flask and condenser are immersed in nitricacid bath (VIV=1:4) for 10 h and rinsed with copious water, then driedin the oven at 60° C. All the other glass stuff, including 20-mL liquidscintillation glass vials and disposable pipets, can be cleaned via thesame procedure.

The recipes include silver nitrate (silver source), PVP (shape-selectivereagent, weight-average molecular weight≈55,000), anhydrous ethyleneglycol (both solvent and reducing agent, such as the product fromAldrich, Milwaukee, Wis.).

A solution of PVP can be prepared by dissolving appropriate amount ofPVP in anhydrous ethylene glycol with the final concentration of 0.375mol/dm³ (in terms of repeating unit). For the preparation of a solutionof silver nitrate, the calculated amount of silver nitrate (milled intofine powder) is added to anhydrous ethylene glycol, then bubbled withair to accelerate the dissolving process. The silver nitrate should bedissolved completely within 1.5 min with final concentration of about0.25 mol/dm³.

To form silver nanocubes, a 5 volume of anhydrous ethylene glycol inflask is heated at 160° C. (in oil bath) for 1 hour. A 3 volume ofsolution of silver nitrate (freshly prepared) and a 3 volume of solutionof PVP are simultaneously added into the hot ethylene glycol by means ofa two-channel syringe pump over a period of 2-12 min. The solution iscontinued to heat at 160° C. for another 10 to 60 min. Vigorous magneticstirring (such as a rotation rate of 400 rpm) is maintained throughoutthe entire process.

The product can be collected via centrifugation. In this case, thereaction product is diluted with acetone (5-10 times by volume) andcentrifuged at 5000 rpm for 15 minutes. The supernatant can be removedusing a pipet and the precipitate is redispersed by adding appropriatesolvents (such as methanol, ethanol, ethylene glycol, water and theirmixtures). In some cases, the product includes some nanorods andnanowires with yield of 5%. The one-dimensional nanostructures with highaspect-ratios can be easily separated from nanocubes through filtration(with Nucleopore® membranes containing pores 1 μm in diameter) becauseof their large difference in dimension.

The nanocubes are single crystals and are characterized by a slightlytruncated shape bounded by {100}, {110} and {111} facets. Their sizeshave narrow distribution.

Example 2

Silver nanocubes were prepared by heating 5 mL anhydrous ethylene glycol(99.8%+, Aldrich, Milwaukee, Wis.) in a 100 mL flask (ChemGlass,Vineland, N.J.) at 160° C. for 1 hour. Two solutions were prepared: a 3mL ethylene glycol solution of silver nitrate (0.25 mol/dm³, 99+%Aldrich) and PVP (0.19 mol/dm3 in terms of repeating unit, Mw≈55,000,Aldrich); and (2) a 3 mL ethylene glycol solution of PVP (0.19 mol/dm³in terms of repeating unit, M_(w)≈55,000, Aldrich). The two solutionswere simultaneously added to the hot ethylene glycol using a two-channelsyringe pump (KDS-200, Stoelting Co., Wood Dale, Ill.) at a rate of0.375 mL/minute. The reaction mixture was then continued with heating at160° C. for 40 minutes. Magnetic stirring at a rate of about 400 rpm wasmaintained through the entire synthesis. The product was dominated bycubic nanoparticles, with a small amount (<5%) of nanostructures withother morphologies (e.g., rods, cubooctahedrons, tetrahedrons, andspheres).

Example 3

Silver nanowires were synthesized via the method described above forExample 2 except that 3 mL ethylene glycol solution of silver nitrate(0.085 mol/dm³) and 3 mL ethylene glycol solution of PVP (0.13 mol/dm³)were simultaneously injected, at an injection rage of 0.375 mL/minute,into 5 mL ethylene glycol. The ethylene glycol had been pre-heated at160° C. before the addition of the solutions of AgNO₃ and PVP.

Example 4

Silver nanoparticles with semi-spherical shapes were synthesized bydissolving 0.025 g silver nitrate and 0.10 g PVP in 10 mL ethyleneglycol. The mixture was then heated at 160° C. for 1.5 hours while itwas vigorously stirred. The silver nanostructures made by the processesdescribed herein could disperse well in water.

Examples 5-13 Formation of Hollow Nanostructures Example 5

The silver nanocubes formed by the method described in Example 1 can beused as sacrificial templates to generate single-crystalline nanoboxesof gold/silver alloy. The resulting gold/silver alloyed nanoboxes arehollow tetradecahedra bounded by six {100} and eight {111} facets. In atypical procedure, a 5 volume of the aqueous dispersion containingsilver nanocubes at a concentration of about 4×109 particles/mL isrefluxed for 10 minutes. A 1.5 volume of 1×10⁻³ mol/dm³ aqueous solutionof chloroauric acid is added drop-wise to the refluxing solution. Thismixture is continuously refluxed until its color became stable. Vigorousmagnetic stirring is also maintained throughout the synthesis.

Example 6

Samples of silver nanocubes were prepared by mixing together (1) a100-μL aliquot of an original dispersion of as-synthesized silvernanocubes, such as those manufactured by the method described above inExample 2, and (2) 5 mL of de-ionized water (purified with cartridgesfrom Millipore, E-pure, Dubuque, Iowa) at room temperature. The diluteddispersion containing silver nanocubes was refluxed for 10 minutes.Aliquots of 1×10⁻³ mol/dm³ HAuCl₄ (99.9%, Aldrich) aqueous solution wereadded dropwise to the refluxing solution. This mixture was continuouslyrefluxed for 20 minutes and the color became stable.

SEM images of the solution were taken following the addition differentvolumes of HAuCl₄ solution to determine the progress of the reaction.Vigorous magnetic stirring was maintained throughout the synthesis. Whenthe solution was cooled down to room temperature, white solid (AgClprecipitate) would settle at the bottom of containers. The AgCl solidcould be removed by dissolving with saturated solution of NaCl (99.9%,Fisher, Fairlawn, N.J.). However, in this example NaCl powders wereadded to the aqueous dispersions of products until the solution wassaturated with NaCl. The solution was then transferred to centrifugetubes and centrifuges at 10,000 rpm for 15 minutes. The supernatantcontaining the dissolved AgCl was easily removed using a pipet. Thesettlings were rinsed with water and centrifuged six times for a time ofabout 10 to about 30 minutes each time. The final solids were dispersedwith water.

Example 7

The reaction described above for Example 6 was repeated with 250-μL and250-μL aliquots for nanowires and spherical nanoparticles, respectivelyto obtain nanoshells in the shape of nanowires and sphericalnanoparticles.

Example 8

Eleven samples were prepared as described above in Example 6. SEM imagesof each of the eleven samples (shown in FIGS. 11A-11K) were preparedafter the addition of differing amounts of HAuCl₄ solution to eachsample. FIGS. 11A-11K show SEM images of the eleven samples of silvernanocubes following the addition of increasing amounts of 1×10⁻³ mol/dm³HAuCl₄ solution. FIG. 11A shows an SEM image sample of silver nanocubesbefore any HAuCl₄ has been added. FIG. 11A illustrates the goodmonodispersity that was achieved using the polyol process describedabove. These silver nanocubes had smooth surfaces and a mean edge lengthof 111 nm, with a standard deviation of 13 nm. The inset shows theelectron diffraction pattern obtained by aligning the electron beamperpendicular to one of the square faces of a cube.

For the samples shown in FIGS. 11B-11K, HAuCl₄, each sample was allowedto react with an amount of 1×10⁻³ mol/dm³ HAuCl₄ solution for 20 minuteswhile vigorous stirring was maintained. FIG. 11B shows an SEM image of asample of silver nanocubes after reaction with 0.05 mL of HAuCl₄solution. As shown in FIG. 11B, small holes had formed in the nanocubes,as indicated by the black spots on the surfaces of the cubes. SEMresults over hundreds of particles indicated that only about one sixthof the particles could display holes. This observation confirms thatonly one hole was formed on each particle since each cube has sixequivalently square faces and any one had the same possibility to sitagainst the substrate. The newly formed surfaces associated with theholes represent the more active sites in further replacement reactions.The particles had no apparent change in size during this initialreaction, indicating that the gold layers deposited on the surfaces werevery thin. The thin gold layers might not be able to well passivate thesurfaces of the silver cubes and thus the further reaction enlarged theholes, as shown in FIG. 11C.

FIGS. 11C and 11D show SEM images of silver nanocubes following reactionwith 0.10 mL (FIG. 11C) and 0.30 (FIG. 11D) of 1×10⁻³ mol/dm³ HAuCl₄solution. The gold layers (or films) would be thickened when more goldatoms were generated by the replacement reaction. Once the gold layerthickness reached a critical value (about 1 nm), the holes would shrinkdue to the volume diffusion, surface diffusion, and/or dissolution anddeposition promoted at 100° C. As shown in FIG. 11D, the openings werereduced as compared to those shown in FIG. 11C. The inset shown in thelower left corner of FIG. 11D gives the image of an individual cube witha slightly higher magnification, exhibiting the coarseness of the holeedge. The formation of extruded structures into the hole implied thatthe mass diffusion process might be account to the shrinkage ofopenings. In addition, the TEM image of a microtomed sample, shown inthe inset in the upper right-hand corner of FIG. 11D, shows that thenanostructures formed at this stage were cubic particles with hollowinteriors. When the volume of HAuCl₄ was high enough, the void sizesincreased to form cubic nanoboxes with uniform walls, which wereconfirmed by the TEM image of a mirotomed nanobox (shown in the inset ofFIG. 11E).

The SEM image in FIG. 11E and the TEM image in the inset of FIG. 11Eshow a sample after reacting with 0.50 mL of 1×10⁻³ mol/dm³ HAuCl₄solution. Note that all the holes had disappeared and form seamlessnanoboxes with smooth, nonporous surfaces. The inset in FIG. 11E shows aTEM image of a mircotomed nanobox. The hollow shape of the nanobox canbe seen in this inset.

FIG. 11F shows an SEM image of a sample formed after reaction with 0.75mL of 1×10⁻³ mol/dm³ HAuCl₄ solution. The nanoboxes in the sample hadtransformed into pinhole-free nanoboxes. The average edge length ofthese nanoboxes slightly increased to 117 from the original average edgelength of the original silver nanocubes (111 nm). This increaseindicates that the generated gold was deposited on the surfaces of thesilver nanocubes. The inset in FIG. 11F shows an electron diffractionpattern which was recorded by directing the electron beam perpendicularto one of the square surfaces. The pattern show in this inset displayedthe same symmetry as that of silver nanocubes, implying an epitaxialrelationship existed between the walls of the nanoboxes and surfaces ofthe silver nanocubes during the replacement process. The corners of somenanoboxes shown in FIG. 11F were slightly truncated, indicating thestart of dealloying process.

FIG. 11G shows an SEM image of a sample of silver nanocubes afterreaction with 1.00 mL of 1×10⁻³ mol/dm³HAuCl₄ solution. The nanoboxesformed were truncated and each one was bounded by two sets of facets(eight triangular faces and six octagonal ones). The inset shows the SEMimage of a truncated nanobox sitting on the silicon substrate againstone of its triangular faces. The crystalline structure of thesetruncated nanoboxes was examined using electron diffraction.

FIGS. 11H and 11I give the electron diffraction patterns recorded fromtwo nanoboxes sitting on the TEM grids against one of their octagonaland triangular faces, respectively. These patterns indicated that eachtruncated nanobox was a single crystal, with its octagonal faces beingbounded by {100} crystallographic planes and triangular ones by {111}planes. As shown in FIG. 11J, the walls of truncated nanoboxes weredecorated with small pinholes of 1-5 nm in size as the volume of 1×10⁻³mol/dm³ HAuCl₄ solution was increased to 1.50 mL. These pinholes wereenlarged during the further dealloying process, as shown in FIG. 11K.The truncated corners bounded by {111} facets disappeared and alsotransformed into large pores. Most pores exhibited the square profile.These porous nanoboxes are referred to as nanocages. Electrondiffraction results suggested that these cubic nanocages were alsosingle crystals bounded by {100} planes.

The nanoboxes of FIG. 11K were formed by reacting a sample of nanocubesas described above with 2.00 mL of 1×10⁻³ mol/dm³ HAuCl₄ solution. Oncethe dimension of these pinholes reached a critical value, they wouldcoalesce into large pores with sizes ranging from 20 to 60 nm, as shownin FIG. 11L. The nanoboxes shown in FIG. 11L were formed by reacting asample of nanocubes as described above with 2.25 mL of 1×10⁻³ mol/dm³HAuCl₄ solution. The nanocages collapsed into gold fragments when thesilver nanocubes had reacted with 2.5 mL of 1×10⁻³ mol/dm³ HAuCl₄solution, shown in FIG. 11M.

Gold/silver alloyed nanoshells such as those formed by the methodsdescribed herein were found to be about seven times more sensitive toenvironmental change when compared with solid gold colloids havingroughly the same diameters. Based on the Mie scattering theory, the SPRband of a metal nanoparticle is expected to red-shift with increasingthe refractive index of the dispersion medium. The high sensitivity ofgold/silver alloyed nanoshells to environmental change, as well as thehigh extinction coefficient in the red and NIR regions, should makenanoshells an ideal platform to probe biologically binding events whichoccur on the colloid surface.

Example 9

Silver nanowires synthesized via the polyol process represented anotherclass of nanostructures with well-defined shapes. Each nanowire had apentagonal cross section (as shown in the inset of FIG. 12A), fivestraight side edges parallel to its longitudinal axis, five flat sidesurfaces bounded by {100} facets, ten {111} end facets, and a five-foldtwinned crystalline structure. Six 5 mL samples of a silver nanowiredispersion in water at a concentration of 1.2×10⁻³ mol/dm³ (in terms ofsilver atoms) were prepared and allowed to react with different volumesof 1×10⁻³ mol/dm³ HAuCl₄ solution. The TEM and SEM (insets) images ofthe resultant nanostructures are shown in FIGS. 12A-12F. FIG. 12A showsTEM images of silver nanowires before the addition of HAuCl₄ solution.FIG. 12B shows an image of a sample after reaction with 0.3 mL of 1×10⁻³mol/dm³ HAuCl₄ solution; FIG. 12C shows a sample after reaction with 0.6mL of 1×10⁻³ mol/dm³ HAuCl₄ solution, FIG. 12D shows sample afterreaction with 1.5 mL of 1×10⁻³ mol/dm³ HAuCl₄ solution; FIG. 12E shows asample after reaction with 2.3 mL of 1×10⁻³ mol/dm³ HAuCl₄ solution; andFIG. 12F shows a sample after reaction with 3.0 mL of 1×10⁻³ mol/dm³HAuCl₄ solution. When the volume of HAuCl₄ was 0.3 mL, each silvernanowire developed interior cavities (i.e. tubular strips), as shown inFIG. 12B. All the silver nanowires were transformed into nanotubes withsmooth and uniform sheaths (FIG. 12C) when the volume of HAuCl₄ solutionadded was 0.6 mL. These nanotubes inherited the typical morphologicalfeatures of silver nanowires. As shown in the inset, each tube has apentagonal cross section, five straight side edges and five flat sidesurfaces. The electron diffraction patterns taken from individualnanotubes were essentially the same as that of silver nanowires. Theseobservations further confirmed that the elemental gold generated fromthe replacement reaction was epitaxially deposited on the surfaces ofsilver nanostructures. Dealloying of these Au/Ag nanotubes could alsogenerate pinholes on their surfaces (FIGS. 12D and 12E). The pore sizewas dependent on the volume of HAuCl₄ that was added to the dispersionof silver nanowires. Similar to silver nanocubes, the holes withrelatively large sizes (>20 nm) also displayed the square symmetry dueto their side surfaces being bounded by {100} facets. FIG. 12 shows theresult of complete dealloying, with the porous nanotubes collapsed intogold nanoparticles.

Example 10

Six samples of silver microspheres were prepared and reacted withdifferent amount of 1×10⁻³ mol/dm³ HAuCl₄ solution. Each sample was 5 mLand contained 1.5×10¹¹ silver microspheres in water. The TEM and SEM(the insets) images in FIGS. 13A-F shown the typical morphologies of theproducts formed at different stages in the replacement reaction. FIG.13A shows solid silver nanoparticles, before the addition of HAuCl₄solution. FIG. 13B shows hollow nanoparticles following reaction with0.25 mL of 1×10⁻³ mol/dm³ HAuCl₄ solution. The nanoparticles shown inFIG. 13B have holes on their surfaces and small void sizes. FIG. 13Cshows a sample of nanoparticles following reaction with 0.60 mL of1×10⁻³ mol/dm³ HAuCl₄ solution. The nanoparticles shown in FIG. 13C areseamless nanoshells with uniform and homogeneous walls. FIG. 13D showsnanoparticles following reaction with 1.00 mL of 1×10⁻³ mol/dm³ HAuCl₄solution. The nanoparticles shown in FIG. 13 are porous nanoshells withsmall pinholes. FIG. 13E shows nanoparticles following reaction with1.20 mL of 1×10⁻³ mol/dm³ HAuCl₄ solution. As shown in FIG. 13E theresulting particles are nanocages with larger pores. FIG. 13F shows theresult of adding 1.50 mL of 1×10⁻³ mol/dm³ HAuCl₄ solution. As shown inFIG. 13F the resulting particles are fragments of gold.

Example 11

Silver nanowires were used in a replacement reaction with platinum toform hollow nanostructures. A solution of 5 mL of silver nanowires at aconcentration of 1.2×10⁻³ mol/dm³ (in terms of silver atoms) in waterwas prepared. The solution was refluxed with 1 mL of 2.55×10⁻³ mol/dm³aqueous Pt(CH₃COO)₂ solution. By refluxing silver nanowires with thePt(CH₃COO)₂ solution for 30 minutes, platinum/silver nanotubes wereformed with relatively high yields. FIG. 14 shows a TEM image of theresulting Pt nanotubes. Compared with Au/Ag nanotubes, the walls ofPt/Ag nanotubes seemed to be rougher and primarily composed of discretenanoparticles. This different could be attributed to the fact that theOstwald ripening process for Pt/Ag nanoparticles might require arelatively higher temperature due to the higher melting point of thismetal. As a result, the wall of Pt/Ag nanotubes could not effectively bereconstructed to form a highly crystalline structure at the refluxingtemperature (about 100° C.) of an aqueous medium. This problem isexpected to be solved by selecting a solvent with a higher boilingpoint, as long as it does not react with silver nanowires or theplatinum salt and both silver nanowires and the platinum precursor canbe well-dispersed or dissolved in it. For example, 2,4-pentanedione (CASnumber 123-54-6) may be suitable solved for a platinum precursorsolvent.

Example 12

A solution of 5 mL of silver nanowires at a concentration of 1.2×10-3mol/dm³ (in terms of silver atoms) was prepared and reacted with 1 mL of10×10⁻³ mol/dm³ of an aqueous solution of Pd(NO)₂. The solution wasrefluxed for 30 minutes at a temperature of 100° C. Note that thePd(NO₃)₂ solution should be freshly prepared before use to prevent thePd/Ag cations from hydrolysis to form Pd(OH)₂ that will be unable tooxidize Ag to Ag⁺. FIG. 15A shows a TEM image of Pd nanotubes obtainedby this reaction. FIG. 15B shows a typical SEM image of sonicatednanotubes, indicating that their surfaces were still relativelycontinuous, smooth and nonporous. The inset of FIG. 15B shows that thePd/Ag nanotubes were hollow and their walls were uniform in thickness.The Pd/Ag nanotubes could be used to catalyze the Suzuki couplingreaction, the coupling reaction between phenylboronic acid andiodobenzene. The Pd/Ag nanotubes performed well as a catalyst and couldbe easily recovered from the reaction mixture by centrifugation andreused as catalyst for another round of the Suzuku coupling reaction.

Example 13

Hollow silver or silver alloy nanoparticles were generated usingsacrificial templates of Ni/Co alloy. A solution of Ni/Co alloyedparticles was prepared by dissolving 0.2037 g Co(NO₃)2.6H₂O) and 0.3419g NiSO₄.6H₂O in 1,2-propanediol. 0.2 g of NaOH was then added to thesolution. After it was dissolved, 50 μL of 0.5×10⁻³ mol/dm³ AgNO₃solution was added at room temperature. The mechanical stirring was keptthroughout the synthesis. The final product was centrifuged and rinsedwith ethanol, water and acetone. The solid products were redispersed in20 mL of water. Hollow silver nanoparticles were obtained at a yield ofabout 80%. The volumes and concentrations described above for thereplacement reactions for silver to gold may be used in converting Ni/Conanoparticles to silver or its alloy.

Examples 14-16 Formation of Complex Nanoshells and Nanotubes Example 14

To form rattle-like core/shell nanostructures, a solution of Au/Agalloyed solid nanoparticles was prepared. Solid Au/Ag alloyednanoparticles with a gold molar fraction in the range of 0.1-1.0 wereprepared by the following process: An appropriate volume (for example,0.31 mL was added if the gold molar fraction was 0.75) of 3×10⁻² mol/dm³HAuCl₄ was added to 50 mL of water and the solution was then heated toits boiling temperature. To this hot solution was added a sufficientamount of 2×10-2 mol/dm³ aqueous AgNO₃ to bring the total concentrationof silver and gold species (HAuCl₄ and AgNO₃) to 0.25×10⁻³ mol/dm³. Avolume of 1% sodium citrate solution (2.5 mL) was added to the refluxingsolution and allowed to reflux for 30 minutes. The final solution wasthen left to cool to room temperature.

Silver plating solutions were mixed with the dispersion of Au/Ag alloyedsolid nanoparticles, resulting in the formation of silver layers. Au/Agalloyed solid nanoparticles with a gold fraction of 0.75 were used. 15mL of the Au/Ag solution prepared as described above was mixed with 2.5mL of 0.1 mol/dm³ ascorbic acid (99.9+%, Aldrich). Then 2.5 mL of 5×10⁻³mol/dm³ AgNO₃ solution was added to the mixture dropwise. The reactionwas allowed to proceed for 30 minutes with vigorous stirring. Theproducts were centrifuged at a rate of 2500 rpm for 15 minutes. Thesupernatant was removed using a pipet. The settlings were redispersed in15 mL water.

The redispersed settlings were refluxed for 15 minutes and a 40.4 mLaliquot of 1×10⁻³ mol/dm³ HAuCl4 was added to the hot solution dropwise.The mixture was refluxed for 20 minutes. The yield of core/shellstructures obtained by this method was as high as 95%.

Example 15

Multiple walled nanotubes were formed by first synthesizing Ag/Ausingle-walled nanotubes. The single-walled nanotubes were generated bydiluting a 250 μL aliquot of silver nanowires (0.02 mol/dm³ in terms ofsilver atoms) with 5 mL water. After refluxing for 15 minutes, 1 mL ofaqueous 1×10⁻³ mol/dm³ HAuCl₄ solution (99.9%, Aldrich) was addeddropwise, and the mixture continued to reflux for another 10 minutes. Tocoat the nanotubes with silver sheaths, 3 mL of the resulting dispersionof silver nanotubes was mixed with 0.8 mL of ascorbic acid (0.1×10⁻³mol/dm³ 99+%, Aldrich), and 0.8 mL of AgNO₃ solution (5×10⁻³ mol/dm³,99+%, Aldrich) was added dropwise. This electroless plating process wasallowed to proceed for 30 minutes at room temperature and the mixturewas centrifuged at 2000 rpm for 15 minutes to remove the excess ascorbicacid.

For the synthesis of Ag/Au double-walled nanotubes, the Ag-coatednanotubes were redispersed in 5 mL water and used for another round ofreplacement reaction. In this round, a 0.8-mL aliquot was used togenerate double-walled nanotubes of Au/Ag alloy. By adding another roundof the electroless plating of silver (using 1.2 mL of 5×10⁻³ mol/dm³AgNO₃) and replacement reaction (using 1.2 mL of 1×10⁻³ mol/dm³ HAuCl₄solution, triple walled nanotubes of Au/Ag alloy were also synthesized.

FIG. 16A shows SEM images and FIG. 16B shows selected area electrondiffraction (“SAED”) patterns of silver nanowires. FIG. 16C shows SEMimages and FIG. 16D shows SAED patterns of single-walled nanotubes ofAu/Ag alloy. FIG. 16E shows SEM images and FIG. 16F show SAED patternsof double-walled nanotubes of Au/Ag alloy. All samples were sonicatedfor 1 hour to expose the cross section of these on-dimensionalnanostructures. The Ag nanowires had a mean diameter of about 50 nm andthe single walled nanotubes had a similar diameter, with a wallthickness around 7 nm. The inset in 16A is a TEM image of thecross-section of an individual Ag nanowire, indicating that each wirehad a 5-fold twinned structure characterized by five single crystallinesubunits (marked T1, T2, T3, T4, and T5). The SAED patterns shown in16B, 16D and 16F are essentially the same, suggesting that an epitaxialprocess was involved in the morphological evolution from nanowires tonanotubes. FIG. 17 shows an SEM image of triple walled nanotubes ofAu/Ag alloy.

Example 16

For the synthesis of double-walled nanotubes whose outer walls were madeof Pd/Ag alloy and the inner walls made of Au/Ag alloy, an aqueoussolution of Pd(NO₃)₂ (1×10⁻² mol/dm³, 99.9%, Alfa Aesar in Ward Hill,Mass.) was used for the second step of replacement reaction. Vigorousmagnetic stirring was maintained throughout the syntheses. Beforeanalysis the AgCl contaminant in nanotube samples was removed bydissolving with a saturated NaCl solution. The nanotubes were recoveredby centrifuging the suspension at 8000 rpm for 12 minutes, followed byrinsing 5 times with water. SEM images of the nanotubes were obtainedusing a FEI field emission microscope (Sirion XL) operated at 20 kV. Inpreparing the SEM samples, the nanotubes and nanowires were sonicatedfor 1 hour to expose their cross section, and then small droplets oftheir aqueous dispersion were put on silicon substrates.

FIG. 18 shows SEM images of double walled with the constituent materialof inner wall being Au/Ag alloy and the outer wall being Pd/Ag alloy.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

The invention claimed is:
 1. A method of manufacturing metalnanostructures comprising: providing a mixture comprising a metal salt,a polyol, and a capping agent; forming metal nanostructures having adesired shape under reaction conditions configured to yield the desiredshape at a higher percentage than any other nanostructure shape; andseparating the nanostructures having the desired shape fromnanostructures of other shapes, wherein the separating comprisesfiltering nanostructures having the desired shape from nanostructures ofother shapes.
 2. The method of claim 1, wherein the reaction conditionscomprise a concentration of the metal salt in the polyol and a molarratio of the capping agent to the metal salt.
 3. The method of claim 1,wherein the metal salt is a silver salt.
 4. The method of claim 1,wherein the metal salt is silver nitrate.
 5. The method of claim 1,wherein the capping agent is poly(vinyl pyrrolidone) (PVP).
 6. Themethod of claim 1, wherein the polyol is ethylene glycol.
 7. The methodof claim 1, wherein the desired nanostructure shape is selected from thegroup consisting of nanocubes, half-cubes, pyramids, nanowires, andmultiply-twinned particles.